Excel autosum problem in Excel

[dog.fish]

When I autosum a spreadsheet horizontally and autosum the resulting column vertically, it comes to zero (as it should).
When I autosum the same spreadsheet vertically and autosum the resulting row horizontally, I get an error message 9.21E-12 instead of zero.
Any ideas as to what is going on here?
dog.fish

 

[alow]

Without seeing the data, or testing it out, I would assume just a rounding issue. You could put a =round(<yourSUMeq here>,0) and it will round 9.21E-12 to 0. 9.21E-12 is the same as 0.00000000000921 (essentially 0).